1. Field of the Invention
The present invention relates to an image display method and an image display program for visualizing a tubular tissue.
2. Description of the Related Art
The advent of CT (Computed Tomography) and MRI (Magnetic Resonance Imaging) making it possible to observe the internal structure of a human body directly with the development of image processing technology using a computer has led to an innovation in the medical field, and medical diagnosis using a tomographic image of a living body is widely performed. In addition, as an example of technology for visualizing the three-dimensional structure of the inside of a human body which is too complex to be understood with only a tomographic image, volume rendering for drawing an image of a three-dimensional structure directly from three-dimensional digital data of an object obtained by CT has been used in medical diagnosis in recent years.
There are variations of volume rendering other than ray casting, such as MIP (Maximum Intensity Projection) MINIP (Minimum Intensity Projection), MPR (Multi Planar Reconstruction) and CPR (Curved Planar Reconstruction) In addition, two-dimensional slicing image or the like is generally used as two-dimensional image processing.
Volume data used in volume rendering is a three-dimensional array of “voxels”, which are three-dimensional pixels of cubic body. A specific value expressing the characteristic of a voxel is referred to as “voxel value”. The volume data is obtained from a collection of two-dimensional cross-sectional images of a target object, such as a body of a patient. Particularly in a CT image, the voxel value represents x-ray attenuation coefficient at the position covered by the voxel, and each voxel value is referred to as “CT value.”
Ray casting is a known technique for volume rendering. In ray casting, a virtual ray is emitted from a projection plane onto an object, and the light reflected from the inside of the object is computed. A two-dimensional image is generated by calculating the reflected light projected onto each pixel on the projection plane. For example, refer to “Introduction To Volume Rendering”, written by Barthold Lichtenbelt, Randy Crane, and Shaz Naqvi, Hewlett-Packard Professional Books (published in 1998) Chapter 6, pg. 121-138.
FIG. 9 shows a colon displayed by the ray casting method as an example of the visualization of a tubular tissue inside a human body. As shown in FIG. 9, by using the ray casting method, an image in which the three-dimensional structure of the colon is seen through can be formed from a two-dimensional tomographic image data sequentially obtained along a direction perpendicular to a tomographic image plane of the abdomen.
FIGS. 10A and 10B are illustrations showing the case where a masking process is performed on a volume to display only a part of the volume. For example, as shown in FIG. 10A, masking process is performed so that only a partial region of the whole volume data can be displayed as a mask region. Accordingly, the outline of an inner wall surface of the colon can be displayed as shown in FIG. 10B, when masking process is performed which designates a mask region to exclude a volume region blocking the view in front of a region of interest of an image of the colon obtained by the ray casting method. As a result, it is possible to observe and detect a lesion that appears on the inner wall surface such as a polyp.
FIGS. 11A and 11B show other examples of the mask region. As the mask region can be any three-dimensional shape, a surface of the mask region can form an arbitrary section or a plurality of arbitrary sections.
FIGS. 12A and 12B are illustrations showing the case where an arbitrary sectional plane of a volume is displayed by MPR (Multi Planar Reconstruction). For example, as shown in FIG. 12A, an arbitrary sectional plane 102 is extracted from the volume data 101 so that a sectional plane 102 can be displayed. FIG. 12B shows an image of the periphery of the colon displayed by MPR. Incidentally, the black part in FIG. 12B expresses air existing in the lumen of the colon.
Next, technical terms for regions concerning a tubular tissue will be described with reference to FIG. 13. As for a tubular tissue 111 such as a colon inside of a human body, a region numbered 112 is called “lumen”, which is filled by air and/or fluid. A wall surface numbered 113 is called “inner wall surface”, on which, for instance, a polyp may be seen. A region numbered 114 is called “inside of wall”, and a region numbered 115 is called “inside and periphery of wall”. The inside of wall constitutes the tissue itself. The periphery of wall includes neighboring tissues and substances surrounding the tubular tissue. Accordingly, the portion displayed by the conventional ray casting is “inner wall surface”, which is generally referred to as boundary surface, whereas the portion displayed by MPR is “inside of wall”, which is a solid portion of the volume.
Ray casting allows viewing the external form of a tubular tissue. However, it is not suitable for displaying the inside of wall of the tissue.
In the case where a masking process is performed so that only a part of the volume is displayed, the outline of the tubular tissue including the inner wall surface can be observed, but the inside and periphery of the wall are hardly recognized. On the other hand, in the case where an arbitrary sectional plane of a volume is displayed by MPR, the outline of the tubular tissue including the inner wall surface is hardly recognized, though the inside and periphery of the wall of the tubular tissue can be observed. In addition, when a tubular tissue having many curvatures such as a colon or a blood vessel is sliced by a plane, the tubular tissue cannot be inspected easily because it is difficult to understand the positional relation between the tubular tissue and a planar image by observing them simultaneously.
For this reason, it is difficult to observe and detect a lesion that appears in the inside of the intestinal wall such as an infiltrating tumor. It is also difficult to understand the position of the lesion. Moreover, it requires additional diagnosis apart from the diagnosis of the polyp.